Mobile radio communications systems are well known and are generally referred to as communications systems that have a plurality of mobile stations that communicate via infrastructure or directly to one another. The infrastructure includes a base station and a system controller. The base station and each mobile station include a receiver and a transmitter for communicating to other base stations or mobile stations.
Mobile radio communications systems make use of radio frequency communications channels for communication between mobile stations and/or base stations. Mobile radio communications systems that make use of frequency division duplex (FDD) or time division multiple access (TDMA) as access modes are well known.
For a centrally controlled TDMA system, the mobile stations in the system will have access to a number of radio frequency carriers or carrier frequency pairs for communication, one or more of which may be available at each base site of the system. The carriers support control and traffic channels.
In TDMA communications systems a communication frequency or frequencies are divided into time frames each of which is divided further into time slots. Mobile stations may be assigned time slots to support their communication. The mobile stations communicate on their respective frequency at their respective assigned time slot(s) as assigned by the system controller of the radio communications system. A particular frequency and time slot may be referred to as a communications resource.
Since the radio frequency spectrum is a scarce resource and competition for spectrum allocation is increasing there is a greater need for radio communications system providers to make more efficient use of the limited radio frequencies that are allocated. Radio communications system providers must look at ways to make efficient use of their respective small allocated section of the radio spectrum through their ability to efficiently assign and reallocate communications resources.
Furthermore, there is continuous pressure on radio communications system providers to constantly improve quality on communications links. For example, the quality of communications signals between users needs to be improved by reducing the effects of interference and fading as much as possible.
Particularly, in TDMA systems when there are low user speeds, such as a pedestrian user, the resulting long fade durations can become a problem. Since the mobile user is moving slowly, or possibly not at all, the quality of communications of services that are delay constrained, such as speech and video, suffer. In GSM (Global System for Mobile Communications) systems, frequency hopping, with coding and interleaving may be implemented so that the communications is conducted over many frequency carriers to reduce the problems of long fade durations. Another approach for reducing fading problems is to implement automatic request for re-transmission with additional error protection on the repeats as described by Esa Malkamaki in "Burst-Level ARQ--An Adaptive Low Delay Error Protection Scheme for Speech Transmission in a TDMA System" presented at the Seventh IEE European Confererence on Mobile and Personal Communications, Brighton, December 1993, which has been discussed for the ATDMA research program for advancing TDMA communications systems. However, both frequency hopping and automatic request for re-transmission consume additional capacity because of the coding needed to implement either approach.
Frequency diversity allows the system controller of the radio communications system to allocate more than one radio frequency to a receiver and transmitter pair and allows the receiver to switch to a new radio frequency on the detection of poor link quality. However, for traditional frequency diversity the same information is being transmitted on two or more frequencies thus consuming valuable communications resources or system capacity.
Thus, it is desired to have a method of improving a quality of communications between a receiver and a transmitter without consuming system capacity or increasing the use of valuable communications resources.